1. Field of the Invention
The present invention relates to an image sensor and a fabricating method thereof. More particularly, the present invention relates to a complementary metal oxide semiconductor (CMOS) image sensor and a fabricating method thereof.
2. Description of Related Art
The process of forming a complementary metal oxide semiconductor (CMOS) image sensor is compatible to the process of forming a complementary metal oxide semiconductor (CMOS) transistor. Consequently, CMOS image sensors and CMOS transistors can be fabricated with other peripheral circuits on the same chip. Thus, the power consumption and the fabricating cost of image sensors can be significantly reduced. In recent years, the CMOS image sensor has replaced the charge-coupled device (CCD) in the lower level application and has become popular day by day.
The CMOS image sensor is composed of a photo sensitive diode and a plurality of transistors. The photo sensitive diode is formed of a p-n junction including an n type doped region and the p type substrate. The transistors are n type transistors having an n type gate (n-poly NMOS). Currently, the CMOS image sensor includes two configurations of 3-T and 4-T. The 3-T configuration means the CMOS image sensor includes a reset transistor (Rx), a source coupling transistor (Dx), a select transistor (Sx) and a photo sensitive diode, while the 4-T configuration means the CMOS image sensor has a transfer transistor (Tx), a reset transistor, a source coupling transistor, a select transistor and a photo sensitive diode. Recently, the CMOS image sensor usually has a disadvantage of current leakage. Generally, the photo sensitive diode and the transistors of the CMOS image sensor would generate current leakage. The problem of current leakage as above mentioned would cause large dark current in the CMOS image sensor, such that the read noise is increased and the image quality is deteriorated, and thus the device performance is decreased.
In particular, for the 4-T CMOS image sensor, the dark current in the CMOS image sensor occurs due to the current leakage of the transfer transistor. Therefore, how to reduce the current leakage of the image sensor becomes an important development topic.